Disposable food containers are well known in the art. Typically, such containers are made from paper or plastic.
Pressed paperboard containers may be made as noted in one or more of U.S. Pat. No. 4,606,496 entitled “Rigid Paperboard Container” of R. P. Marx et al; U.S. Pat. No. 4,609,140 entitled “Rigid Paperboard Container and Method and Apparatus for Producing Same” of G. J. Van Handel et al; U.S. Pat. No. 4,721,499 entitled “Method of Producing a Rigid Paperboard Container” of R. P. Marx et al; U.S. Pat. No. 4,721,500 entitled “Method of Forming a Rigid Paper-Board Container” of G. J. Van Handel et al; and U.S. Pat. No. 5,203,491 entitled “Bake-In Press-Formed Container” of R. P. Marx et al. Equipment and methods for making paperboard containers are also disclosed in U.S. Pat. No. 4,781,566 entitled “Apparatus and Related Method for Aligning Irregular Blanks Relative to a Die Half” of A. F. Rossi et al; U.S. Pat. No. 4,832,676 entitled “Method and Apparatus for Forming Paperboard Containers” of A. D. Johns et al; and U.S. Pat. No. 5,249,946 entitled “Plate Forming Die Set” of R. P. Marx et al.
Thermoformed plastic containers, particularly polypropylene mica-filled containers with a micronodular surface are disclosed in U.S. Pat. No. 6,100,512 to Neculescu et al. Such containers have the advantages that they are durable and may be washed and re-used if so desired and are microwaveable. The disclosure of the foregoing patents is incorporated by reference.
A drawback of plastic thermoformed containers is that they tend to be more costly than their paper counterparts due, in part, to material costs. More rigid materials can be used more sparingly and are thus highly desirable in the field. One way to increase rigidity of polypropylene containers is to use a filler such as mica as disclosed in the '512 patent. However, mica tends to interact with polypropylene to produce undesirable odors, believed to be caused by certain organic ketone compounds generated during melt-processing of the material. The generation of odors is minimized by including a basic inorganic compound, such as calcium carbonate in the composition.
It has been unexpectedly found that fine grades of calcium carbonate can greatly increase the rigidity of polypropylene containers filled with calcium carbonate and mica as described hereinafter.